It is known that vacuum-type circuit interrupters generally comprise an evacuated insulated envelope with separable contacts disposed within the insulated envelope. The contacts are movable between a closed position of the circuit interrupter in which the contacts are firmly engaged and in an open position of the circuit interrupter when the contacts are separated to establish an arc gap therebetween. During separation of the contacts, it is known, that a resulting arc discharges arc erosion products from the electrodes emitting and depositing these undesirable products on inner surface walls and arc shields are provided for that purpose. Vacuum-type circuit interrupters are disclosed in U.S. Pat. No. 4,210,790 in which a pair of relatively opposed electrodes are provided with each electrode having an annular contact making section.
Another vacuum-type circuit interrupter is disclosed in U.S. Pat. No. 3,997,748 in which the pair of primary electrodes are protected and surrounded by a shield from electrode erosion and weld forces. Other related vacuum-type circuit interrupter matters are disclosed in U.S. Pat. Nos. 4,431,885; 4,408,107; 4,414,448; 4,481,390; 4,394,554; and 4,345,126.
It is generally known that the arc shield prevents metal vapor emitted from the arcing region between the electrodes from depositing on an insulating wall of a vacuum-envelope. For high voltage vacuum circuit interrupters, the arc shield is electrically floating and the construction of the electrodes and contacts are symmetrical. However, as vacuum-type circuit interrupters are made smaller in length and diameter, a low cost construction is used in which the arc shield is tied to one of the end flanges of the circuit interrupter and one of the electrodes is placed close to this end of the circuit interrupter.
In the Journal of Applied Physics, Volume 40, pages 1744 to 1752, March, 1969, C. W. Kimblin has in an article entitled "Anode Voltage Drop and Anode Spot Formation in D.C. Vacuum Arcs" has discussed and shown that the current at which an anode spot will be formed is directly related to the anode arcing area for a given electrode separation. Consequently, the larger the anode area, the higher the arcing current must be before an anode spot will occur. Uncontrollable anode spots cause gross erosion of the electrodes and will deteriorate the current interruption level.